Particle blast cleaning apparatus are well known in the industry. While sandblasting equipment is widely used for many applications, it has been found that the utilization of particles which naturally sublimate can advantageously be utilized as a particulate media of such equipment to minimize adverse environmental results and cleanup required following the cleaning activity.
Earlier particle blast cleaning apparatus utilizing subliminal particles have included a rotary transport and more recently a lateral slide bar transport. An example of the rotary transport may be found in U.S. Pat. No. 4,617,064, which issued to the present inventor Moore on Oct. 14, 1986. It discloses a particle blast cleaning apparatus utilizing carbon dioxide pellets in a high pressure carrier gas. The particular particle blast apparatus described in the Moore '064 patent includes a body which houses a rotary pellet transport mechanism having transport bores used to convey the carbon dioxide pellets from a gravity feed storage hopper to the high pressure carrier gas stream for transportation of the pellets to a discharge nozzle.
While the apparatus and method described in the Moore '064 patent can be utilized to accomplish particle blast cleaning, there are some very important practical problems. One significant problem associated with this apparatus is the agglomeration of the pellets when exposed to moisture. This moisture can be introduced into the system from the high pressure carrier gas stream through the discharging station. For this reason it is important to effectively seal out the moisture contained in the high pressure gas stream. In order to ensure that the high pressure gas does not leak into the rotary transport apparatus, a rather complex system of variable pressure gas seals is necessary.
In the Moore '064 reference, the rotary apparatus is fitted with a corresponding set of circular face seals, and means to establish a force on such seals which is proportional in magnitude to the pressure of the transport gas. In order to achieve and maintain this critical sealing function, the circular seals must remain substantially flat in order to remain in intimate, continuous contact with the surfaces to be sealed. In addition to the manufacture of the rotor, a significant amount of machining is required to the housing that the rotary transport is disposed in. These factors contribute to a relatively high fabrication cost of the rotary transport unit.
As a result of the force required to be exerted on the seals, the sealing surfaces must withstand a relatively great amount of friction, with such friction being applied at varying rubbing velocities across the diameter of such circular seals. The rubbing velocity and friction differentials tend to wear the seals at correspondingly different rates, creating a relatively difficult seal maintenance problem. Additionally, it has been found that the seal surface becomes subjected to erosion in critical sealing areas adjacent the receiving station due to occasional shearing of the particulate media at the cavity/receiving station interface.
These seal maintenance problems led to the icing of the rotor surface due to the low temperature and slight residual moisture of the air supply which further degrades the seal, thereby allowing additional moist air to leak into the system. Empirically, it has been observed that the system under the Moore '064 patent cannot operate at discharge air pressures above approximately 175 psig without causing significant leakage of moist air into the apparatus. In order to provide delivery of the particulate media at a sufficient velocity from the nozzle, it is necessary that the apparatus be capable of handling higher discharge air pressures.
It was also found that the apparatus design results in a slight time delay between successive discharge of pellets from the transport means. This causes a non-uniform or pulsating discharge of the particulate media from the apparatus. Additional rotary mechanisms which could be added using the Moore '064 design present a relatively complex and expensive modification problem. Maintenance problems would, of course, be correspondingly multiplied with the addition of more transport means.
Present inventors Moore and Crane have been issued U.S. Pat. No. 4,744,181 for a Particle-Blast Cleaning Apparatus and Method. The Moore '181 Patent discloses a lateral transport apparatus, which offers certain advantages over the rotary transport method. However, several drawbacks remain with the apparatus disclosed therein. In the lateral transport apparatus a plurality of sliding bars, each having a transport cavity which is alternatively alignable with a receiving station and a discharge station, is disposed within channels located in a housing. As each individual bar reciprocates laterally, the corresponding transport cavity is brought alternatively into alignment with the receiving station, at which position pellets are gravity fed into the transport cavity, or with the discharge station, at which position the pellets are discharged by the high pressure carrier gas stream for transportation of the pellets to the discharge nozzle. The relative positioning of each transport cavity is synchronized such that the time delay between successive discharges of pellets from the nozzle is minimized.
With the lateral transport apparatus, it also is necessary to maintain a seal between the upper and lower surfaces of the slide bar to prevent moist air of the high pressure carrier gas stream from leaking into the transport apparatus. Here again, face seals are used to seal between the sliding bar and the housing. It has been discovered that close tolerances are required to maintain the necessary flatness of the mating parts. This problem of sealing is multiplied by the use of the plurality of slide bars disclosed in the application.
The increased number of moving parts, combined with the close tolerances required, results in a design that is both expensive to manufacture and to maintain. Also, by increasing the number of sliding parts which are sealed, the frictional losses of the unit are correspondingly increased. Empirically it has been determined that this system will not operate at discharge pressures above approximately 125 psig, because it requires additional drive power due to excess seal friction. This further limits the ability to obtain the required airflow velocity necessary to maximize the effectiveness of the cleaning apparatus.
Both the Moore '064 and Moore et al '181 patents use only the action of gravity for transporting the pellets from the storage hooper to the transport cavities. It has been observed that the gravity feed by itself produces less than optimum flow to the transport cavity, resulting in only a partial fill of the cavity. In order to obtain a complete fill of the cavity using only gravity feed, it is necessary to increase the dwell time of the transport cavity at the receiving station. The result of increasing the dwell time is a decrease in the delivery frequency of the particulate media to the discharge station, thereby decreasing the delivery of the media to the nozzle and subsequently to the work piece. Thus the operator is faced with the choice between one frequency of delivery of a quantity of pellets which only partial fills the transport cavity, or a lower frequency of delivery of a greater quantity of pellets which completely fills the transport cavity. While gravity flow of the pellets to the transport cavities can be used to deliver pellets to the transport gas flow and subsequently to the work piece, it results in the delivery of less than the optimum quantity of pellets to the work piece.
Despite the prior work done in this area, there remain problems of improving the reliability and cost of achieving and maintaining a proper seal between the particulate media transporting apparatus and the high pressure conveying gas required to discharge such particulate media. Additionally, there remained problems with achieving a relatively uniform delivery of sublimable particulate media in an economical and relatively simple manner. Consequently, prior art structures and processes delivered a relatively inefficient system with rather high maintenance costs.